11.5 Streams and Stream Flow

A stream is a body of flowing surface water of any size, ranging from a tiny trickle to a mighty river. The area from which the water flows to form a stream is its drainage basin. Some of  the precipitation (rain or snow) that falls within a drainage basin is returned to the atmosphere via evaporation or transpiration. The rest either flows across the surface and into a stream, or infiltrates to become groundwater and then slowly flows towards a stream (although it is possible that some of that groundwater will cross into an adjacent drainage basin via groundwater flow). An example of a drainage basin is shown in Figure 11.5.1.

 

The hydrograph of Figure 11.5.3 is based on data from the marked gaging station. The Ashnola River occupies a drainage basin of approximately 1000 km² within a glaciated alpine region of southern British Columbia and northern Washington. There are numerous peaks above 2500 m within the basin (Figure 11.5.2).

As shown in Figures 11.5.2 and 11.5.3, the upper and lower parts of the Ashnola River have quite steep gradients (in the order of 50 m/km, more in some areas) while the middle part and the part within the valley of the Similkameen River, are relatively flat (6 m/km, or less). The shape of the valley has been controlled first by tectonic uplift (related to plate convergence), then by pre-glacial stream erosion and mass wasting, then by several episodes of glacial erosion, and finally by post-glacial stream erosion and more mass wasting. The lowest elevation of the Ashnola River is 440 metres at the Similkameen River, and that is its base level. The Ashnola River cannot erode below that level unless the Similkameen River erodes deeper into its flood plain (the area that is inundated during a flood).

Rivers represent excellent sources of water, but it is critical to understand their flow characteristics in order to be able to manage the supply against the demand. Discharge from a stream is typically displayed in a hydrograph, where the stream flow rate is plotted on the y – axis and the date is plotted on the x axis.  The flow rate can be from single measurements at specific intervals (e.g., hourly), or can be averaged over daily, weekly or even monthly intervals.  Hydrographs reflect the timing of the input of rainfall to a drainage basin, and the timing of the melting of snow and glacial ice.

A hydrograph of daily average discharge levels (flow volume) for the Ashnola River is shown on Figure 11.5.4. As is typical for a stream in a relatively dry climate with cold winters, the hydrograph is dominated by spring and summer snow melt. The discharge ranges from 1 to 3 m³/s in the fall and winter months when much of the drainage basin is frozen and almost all of the precipitation is falling as snow, to over 50 m³/s in the spring, when the snow is melting rapidly and there is some rainfall. The snow-melt discharge decreases gradually from May through August, with a few minor peaks that are likely related to summer rains.

The Englishman River on Vancouver Island is situated in a warm and wet climate, where most precipitation comes as rain in the autumn and winter and the summers are dry.  There is snow accumulation in the upper parts of the drainage basin but most of the area is below 400 m, and very little snow lasts beyond April. The hydrograph (Figure 11.5.5) shows consistently low flows in the summer and strong autumn and winter peaks related to both steady winter rain and to specific rainstorm events.

The two rivers above both demonstrate periods of the year with ample river flow, and periods with low flow.  If these rivers were to be used as water supply, the periods of low flow could likely only sustain limited extraction of water before unacceptable stress was put on the ecosystem.

All of these basins have large reservoirs that have been created by the building of dams.  The reservoirs provide water storage for times when the flow in the streams is low. As shown on Figure 11.5.7, the discharge on the Capilano River where it enters the Capilano Reservoir is similar to that of the Englishman River on Vancouver Island, but also has some sustained summer flow, like the Ashnola River in the interior. The Capilano basin is surrounded by peaks in the 1500 m range, and much of its basin is above 800 m. These higher-altitude areas accumulate thick snow that provides significant meltwater flow into early July.

The Metro Vancouver watersheds are situated in a wilderness area that is now off-limits to the public—even for recreation—and also to industry. There has been logging in those areas in the past, as can be seen from cut-blocks on Figure 11.5.6, but that is no longer permitted. This ensures that the water entering the system is as free as possible from human contamination.  The water supply still needs to be filtered because at some times of the year the heavy flow of the incoming streams makes the water cloudy with suspended clays, and suspended matter limits the effectiveness of disinfection measures such as UV light, ozone or chlorine.

Surface water supplies for many other cities are not as pristine as Vancouver’s because there may be industry, agriculture, recreation and human habitation within the watersheds. In such cases much greater care needs to be taken to check for industrial toxins, farming chemicals and animal wastes, highway runoff, human wastes and effluent from landfills, and to remove any such contaminants.